Electron-emitting device and method of preparation



Jan. 19, 1932. M. N. RICH ET AL 1,842,293

ELECTRON EMITTING DEVICE AND METHOD OF PREPARATION Filed May 19, 1926INVENTOR M/QLCOLM N. RICH Patented Jan. 19, 1932 uNrrao STTES;

mALcoLm N. men mp ERNEST molt LEDERER, or ms'r omen, NEW JERSEY, AS-srenons TO WESTINGHOUSE LAMP COMPANY, A coaroaanon or PENNSYLVANIAELEGQRON-EMITTING DEVICE METHOD OF PREPARATION Application filed May 19,1926. Serial No. 110,047.

7 to yield upon proper treatment, a low temperature electron emission ofgreat utility, which is attributed to the formation of a surface atomiclayer of thorium metal upon the filament.

Various methods have been devised, heretofore, for the purpose ofeffecting a conversion of the thorium oxide content of drawnthoriated-tungsten filaments, to metallic thorium, so that the filamentsmight be advantageouslyused as cathodes in thermionic valves. One of the.common methods heretofore employed, depends upon the addition of acarbon reducing agent to the filament in one form or another, for thepurpose of effecting a reduction of the thorium oxide or suboxidecontent of the filament to thorium metal, during a subsequent seasoningschedule given the filament after it has been mounted in the electrondischarge device and the device suitably exhausted. The socalledseasoning of the filament is for the purpose of enabling this addedcarbon to effeet a reduction of the thorium oxide content of thefilament, and the temperatures are so regulated that theliberatedthorium metal is diffused to the surface of the filament.

Such a method is described in application Serial No. 636,510 of Ralph E.Myers, filed May 3. 1923. entitled Evacuated devices and method ofexhaust, and assigned to the West-" inghouse Lamp Company. Briefly, themethod consists in heating the filament in a volatile hydrocarbonatmosphere at such a temperature and time interval, that decom-,position of the hydrocarbon is effected, and

a deposit of carbon is formed upon the fila ment.

This addition of carbon to the filament has been found to be extremelyadvantageous in the manufacture of high power electron dis chargedevices in which the plate electrode is operated at a high temperatureand has also been found advantageous in developing life and electronemission in cathodes used in low power devices under the conditionswhich have heretofore prevailed in their manufacture. However, withimprovements in the manufacturingconditions and with electrode materialsnow available for the manufacture of low wattagetubes, we have foundthat the addition of carbon is not only unnecessary but improved resultsare obtained if the small amount of carbon accumulated during thedrawing and. swaging operations is substantially removed.

Recent investigation in radio tube construction has shown that thematerials used in the construction of the plates and grids exert aprofound influence on the life and maintenance of the thorium emissionof the filament. This is due to what is known as back oxidation effects,which are produced through the liberation of the oxygen contained in theoxide films on the plate and gird metal parts, under electronicbombardment, which oxygen migrates to the filament and destroys thethorium film thereon.

When this oxide film on the plate and grid metal parts was removed byany suitable means, such asdescribed in application Serial No. 18,958 ofH. C. Rentschler, filed March 28, 1925, entitled Electron dischargedevices and assigned to the Westinghouse Lamp Company, or copendingapplication Serial No. 85,016 of E. A. Lederer, et a1, filed January 30,1926, entitled Electrodes for electron discharge devices, and assignedto the Westinghouse Lamp Company, it was found that it was not necessaryto add carbon to the filament to obtain a formation of thorium metal onthe filament but on the contrary an excess quantity of carbon appearedto retard or inhibit the decomposition of the thorium constituent of thedrawn siilly used, is that the added carbon combines with the oxygenliberated from the plate and grid metal parts, and thus prevents theoxidation of the thorium metal film on the filament. The amount ofoxygen or oxide in or on the plate and grid and the extent ofcarbonizatlon of the cathode differ in each particular instance,depending on the treating conditions and can not be uniformlycontrolled. As a consequence, difiiculty has been experienced inobtaining tubes having uniform operating characteristics. If there 1s anexcess of oxide on the plate, loss of emission will result due tooxidation of the thorium content of the filament, whereas, if the carbonis in excess, loss of emission, due possibly to thorium carbideformation, is obtained.

It is an object of this invention to eliminate these variables incidentto the manufact-ure of electron tubes and produce thorium activatedcathode tubes having a high emissivity, long life and uniform operatingcharacteristics. I

Another object is to provide an activated thoriated-tungsten filamentwhich is substantially free from deleterious carbon.

Another object is to provide an improved method of activating cathodesof the thoriated type whereby increased electron emission and life areobtained.

Other objects and advantages will herein.-

after appear.

In the performance of this invention we preferably employ drawnthoriated-tungsten filaments, as prepared by the usual hot workingmethods, such as the Coolidge method, described in Patent 1,082,933,although squirted filaments or tungsten-thorium alloy filaments whichhave been subjected to carbonization might similarly be treated, according to the method hereinafter described.

Briefly, the Coolidge method consists in doping tungsten oxide with a.solution of thorium nitrate such as to yield after drying and ignitingat a temperature sufi'icient to decompose the thorium nitrate, thedesired percent of thorium oxide as a finely disseminated powder.

The tungsten oxide is then reduced in hydrogen, the metal powdercompacted into slugs and the slugs suitably sintered to a coherent metalbody in hydrogen.

Following the treating of the slug it is given the usual swaging anddrawing operations, during which appreciable carbon absorption in themetal body is necessarily encountered, particularly in the highertemperature swaging operations, due to the carbonaceous lubricant andthe gas furnaces employed in these operations.

in accordance with our invention the carbon addition heretoforeemployedto activate the filament is not only entirely dispensed with but we takethe thoriated-tungsten filament at the conclusion of the drawingoperation and remove therefrom a large proportion of the carbides andsurface carbon obtained during fabrication, by subjecting the filamentto the action of especially purified dry hydrogen, at a temperature andtime interval necessary to effect such a removal.

Obviously, such an operation varies with the size of filament, extent ofcarbonization, etc. We have discovered that at temperatures ranging from1500 to 2000 (3., the carbides are decomposed rapidly b hydrogen withthe removal of the carbon rom the filament. If the hydrogen treatment iscontinued for too long a period of time or if the temperature employedis too high, good results will not be obtained, due possibly to partialreduction of the thorium compounds and while this is the ultimate objectin activating the filament, ,it should be prevented until the filamentis enclosed in the evacuated envelope since otherwise a portion of thethorium may be oxidized by contact with the atmosphere or if notoxidized, it will be volatilized out of the wire during the hightemperature flash to which the wire must be subjected after mounting todevelop full electron emission from the filament through thedecomposition of the thorium compound content.

Preferably, the filament is hydrogen treated in a continuous mannerbefore mounting in the electron discharge device or lamp, but ifdesired, the hydrogen might be introduced into the envelope and thefilament heated therein by a passage of current therethrough, thehydrogen being subsequently exhausted. fter such treatment the filamentmay be incorporated into the electron discharge de' vice and the sameexhausted to a high degree, the electrodes being treated to remove ascompletely as possible, all occluded gases. The filament may then beheated to approximately 2850" G. to decompose the thorium compound, thistreatment being continued only for a sufficient period of time to effectsuch reduction, since the thorium metal volatilizes away rapidly at thistemperature.

While we do not desire to be limited to any particular theory regardingthe activating action obtained by our treatment We suggest thefollowing, as a possible explanation.

During the normal hot working of the wire it is subjected to strongcarbonizing influence, due to the gas fired furnaces in which the wireis heated and the carbonaceous lubricant employed so that after thefinal drawing operation the thorium constituent of the wire may be, to acertain extent, converted to a carbide of thorium which must be laterreduced in order to obtain an active thorium metal film on the filamentfor electron emission. Free carbon and tungsten carbide are also formedduring these hot working operations and appear to retard the reductionof the thorium carbide during the socalled seasoning of the filament.Under this theory, in order to obtain a complete reduction of thethorium carbide, it is first necessary to eliminate free carbon andtungsten carbide, or a substantial portion thereof. if an excessquantity thereof has been formed. It is highly desirable, however, thatthe thorium carbide or other thorium compound of thorium be not reducedprior to the seasoning of the filament, since otherwise a largeproportion thereof will bevolatilized from the cathode at thetemperature to which it is necessary to heat the filament to obtainreduction of the main body of thorium compounds.

This selective decomposition of carbon, that is, the decomposition oftungsten,- carbide and removal thereof, together with the free carbon inthe filament. without substantial reduction of the thorium compoundappears to be accomplished by the hydrogen treatment. There are strongindications that in hydrogen the tungsten carbide is reduced morereadily than the thorium compounds so that if the temperature and periodof the hydrogen fiashingiis correctly chosen, a very substantialproportion of the tungsten carbide will be removed without appreciablereduction of the thorium compounds. It further appears that in vacuo thethorium compounds are decomposed more readily as a result of thehydrogen treatment previ ously given, so that a large available supplyof active thorium metal is produced in the filament.

\Ve have found if the heating in hydrogen is conducted at too high atemperature or for too long a period, that the emission and life of thecathode is detrimentally aflected, due possibly to the reduction of alarge proportion of the thorium compound in addition to the tungstencarbide, the thorium metal so produced being volatilized away during thesubsequent seasoning of the cathode, thus reducing the available thoriumcontent.

In order that the invention may be more fully understood, reference willbe had to the accompanying drawing in which the single figureillustrates diagrammatically apparatus which may be employed fortreating the filament.

The thoriated-tungsten wire 1 as it comes from the final drawingoperation, is wound on a reel 2 which may be mounted upon a suitablestandard 3. The wire is withdrawn from the reel 2 and rewound on asecond reel 4 mounted on a standard 5 and provided with suitable windingmechanism, such as a pulley 6 and a belt 7. A slight amount of tensionis applied to the wire as it is drawn from the reel 2 by any suitablebraking mechanism, such as a friction disc 8.

The wire on its passage from the reel 4 may pass through a tube ortrough 9 open to the atmosphere and supported on standards and outletpipe 17.

10. A mercury cup 11 at the entrance of the tube 9 makes electricalconnection with the wire from a suitable source of current 12 through aconductor 13. A second mercury contact 14 completes 'the circuit throughthe wire and permits it to be heated during its passage through the tube9. This heating'of the wire in air or in an oxidizing atmosphere exertsa beneficial effect on the filament, rendering it soft and more pliable,and removes a large proportion of surface carbon.

The wire as it leaves the tube 9, is drawn through the hydrogen treatingchamber 15 in which it is subjected to the treatment which forms thesubject matter of this application. A stream of pure dry hydrogen issupplied to the chamber through the inlet pipe 16 Electrical connectionsfor heating the wire during its passage through the hydrogen chamber isprovidedby mercury contacts 18 secured to the opposite end walls of thechamber. The mercury contained in contacts 18 seal the openings 19 inthe end walls of the chamber and prevent the entrance of air therein. Apressure of hydrogen of approximately 7 millimeters of water ismaintained within the chamber to create an internal pressure whichassists in excluding the entrance of air therein. Heatmg current for thewire is supplied from the electrical source 12 by conductors 20 and 21,i

the voltage being adjusted by a suitable rheostat 22. A second rheostat23 may be used to control the heating of the wire during its passagethrough the annealing chamber 9.

The temperature to which the wire should be heated during the hydrogentreatment J depends upon the amount of carbon-which it has absorbedduring the wire drawing process and varies with different samples ofWire. We have found, however, that the proper temperature necessary toaccomplish the reduction of the carbides and removal of free carbon, maybe determined by a simple mechanical test. Wire, as it comes from the.drawing dies, tends to curland kink. An arbitrary rate of speed may beselected, .de-

pending upon the size of the wire to be treat- F ed or distance betweenthe mercury contacts 18 etc., which from previous experience has beenfound to be approximately the proper speed and the temperature thengradually raised by adjusting the rheostat 22 until the wire or filamentcomes out of the hydrogen chamber bright and clean and free from kinksand curls normally experienced.

In the apparatusdescribed. the distance between the mercury contacts 18was approximately 2 inches and the speed with which the wire was passedthrough the chamber varied from 1 to 3 inches per second depending uponthe size of the wire. For wire size of approximately .6 mil whichcorresponds to a wire having a weight of .71 to .74 milligrams per 200millimeter length, a speed of 2.3 to 3 inches per second was employedand the voltage across the ends of the section of wire undergoingtreatment was adjusted between and volts. A gas flow of 3 to 7 cubicfeet per hour Was employed. In case of 1 mil wire at the speed givenabove, voltages between 35 and 48 volts were applied across the ends ofthe section undergoing treatment. In wire of approximately 1.6 mil aspeed of 1% to 2 inches per second was employed with a potential betweenthe ends ofthe section undergoing treatment of 18 to'28 volts.

The flashing in hydrogen must be carefully conducted since ithas beenfound that hydrogen exerts a profound embrittling of feet on tungsten.It has been the practice, heretofore, to treat tungsten filaments inhydrogen, but such treatment has been for the purpose of annealing orrecrystallizing the wire. Hydrogen was employed because it was believedto be inert with respect to the filament and the flashing done withoutregard to the physical efi'ect of the absorbed hydrogen on the finishedfilament or the development of electron emission in the filament. I

In accordance with the present invention the treatment in hydrogen isconducted for a very short period of time, the best results having beenobtained when the wire was subjected to the reducing action of thehydrogen from about .5 to 1.5 seconds. This period of time is sufiicientto effect a substantial removal of carbon from the filament with verylittle or no decomposition of thorium compounds. hydrogen absorbed bythe wire during this short interval of heating is not sufiicient toembrittle the same. and the recrystallization which takes place duringthis brief heat treatment is insufiicient to materially reduce the strenth of the filament.

We have further found it essential to employ a verv pure dry hydrogen,since the presence of any moisture in the gas deleriouslv effects theemission of the filament. The hydrogen employed in our process ispreferably run through a pre-heater and throu h sulphuric acid,potassium hydroxide and phosphorus pentoxide driers to obtain the properpurity and dryness.

Following the usual process of mounting, sealing-in, exhausting, andflashing the getter of the electron discharge device, the filament isseasoned at a temperaturein excess of 2500 C. and at approximately 2850C. to effect a substantial conversion of the thorium compound content ofthe wire to thorium metal, in whatever manner it is accomplished, i. e.,decomposition, dissociation, diffusion or reduction, and the thoriumsurface film built up on the filament in the usual manner at atemperature around 2200 C. For op- Moreover, the quantity ofv orationthe filament is heated to approximately 1700 C.

With electron discharge devices of the {IX-199 type which employfilaments of approximately .6 mil diameter, the temperature foreffecting the decomposition of the thorium carbide is obtained byapplying a voltage of 11.5 to 12 volts. The temperature for difiusion ofthe thorium to the surface is obtained with 1.5 volts. The operatingvoltage is 3.3 volts.

With electron discharge devices of the 'UX-QOLA type which employ afilament of approximately 1.6 mil diameter, the following seasoningprocedure is employed. Decomposition, of the thorium compound isobtained with 17 to 19 volts and diffusion to the surface with '15 to8.0 volts. The normal operating voltage is 5 volts.

Electron discharge tubes employing filaments treated in accordancewithour invention, taking precautions to remove a sub stantial proportion ofthe carbon content ol tained during the drawing operation and ohservinggood exhaust and oxide free plates and grids, give uniformly highinitial electron emission and maintain such emission over a longerperiod of time than heretofore attained with tubes of similar design.

While this filament has been referred to in connection with oxide freeplates and grids, it is not limited to use with such electrodes. but dueto the fact that there is available moremetallic thorium than hasheretofore been obtainable, increased emission and life.

are procured with ordinary plate and grid materials. The oxide freeplates and grids, however, tend toward the production of uniformity inthe product, decreased shrinkage due to emission failures and increasedefiiciency and prolonged life of the device.

Forming gas, a non-explosive mixture of 10% hydrogen and 90% nitrogen,suitably dried and purified, may be advantageously used in this processinstead of pure hydrogen, with slight adjustment in time andtemperatures. to accommodate the process to the lower hydrogen content.

In order to render the filament more uniform in cross sectional area andto eliminate hot spots and irregularities therefrom, and to moreeffectively remove the carbon, the wire just prior to drawing or at anyconvenient point during the drawing operation, may have the surfacescale of carbon, carbides, oxides, etc., removed and the wire renderedpliable by the treatment set forth in copending applications of MalcolmN. Rich, Serial No. 4140-, filed January 23, 1925, entitled Pliabletungsten and method of producing the same; and Serial No. 65,881, filedOctober 30, 1925, Patent No. 1,663,564 entitled Refractory metalfilament, both assigned to the Westinghouse Lamp Company.

Briefly, such process consists in making which evolves oxygen at theanode and isolating the cathode therefrom so that the hydrogen thereliberated does not come in contact with the anode. This treatmentremoves the carbon and surface carbides and by eliminating the hydrogenfromrthe wire, renders it extremely pliable at room temperatures. WViresubjected to this anodic treatment to remove the major portion of thecarbon therefrom during drawing and then subjected to the carefulhydrogen treatment specified above, after completion of the drawing hasbeen found to give still greater life and maintenance for use inincandescent lamps than the hydrogen treatment alone and also to improvethe product for use as cathodes in electron discharge devices. However,such anodic treatment is not essential to the production of excellentresults but is merely augmentive in character.

While this invention contemplates the removal of carbon to assist in theactivation of the filament, nevertheless, the addition of carbon to thefilament as heretofore practiced does not necessarily preclude thedevelopment of some electron emission from the filament in an electrondischarge device, particularly in power tubes in which high platevoltages are employed or in low voltage tubes if the device has beeninsufficiently evacuated, or if the plate and grid metal parts are notoxide free. However, the results obtained are not uniform. The amount ofthorium metal reserve in the filament may be negligible, or it may besufficient to maintain fairly good life.

On the other hand elimination of carbon from the filament does notnecessarily ex ose the filament to unusual destruction rom oxidizinginfluences, because of the fact that due to the greater thorium metalreserve formed in the filament by the hydrogen treat-- ment, the normaloxidizing conditions encountered, in electron dischar e devices, are notsufficient to appreciably e eat the initial electron emission of thefilament nor the total operating life of the device.

Obviously, man changes may be made in the method dcseri ed and otherapplications of the invention devised without departing from theinvention and we do not desire to be restricted to the particularembodiments described except as limited by the appended claims.

, What is claimed is:

1. The method of 'producin activated thoriated-tungsten cathodes whichave been subjected to carbonizing influence, com rising heating thecathodes below 2000 in hydrogen to substantially remove the carbon andtungsten carbide contentthereof and subsequently heating the filament invacuo at about 2850 C. to decompose the thorium compounds therein.

2. The method of activating a thoriated tungsten cathode which comprisesheating the cathode to from 1500 to 2000 C. in dry hydrogen for aboutone second and subsequently heating the cathode to about 2850 C.in'vacuo for about one minute. Y

3. The method of activating, for electro emission purposes, a thoriatedtungsten filament which has been subjected to carbonization whichconsists in removing surface carbon and other impurities from thefilament prior to final drawing by electrolytic action and after finaldrawing heating the filament in dry hydrogen to from 1500 G. to 2000 C.for a brief period to further eliminate carbon therefrom and thereafterheating the filament in vacuo to temperatures approximating 2850 C.followed by a second and longer heating in the same .vacuo totemperatures approximating 2200 C.

4. The method of activating a thoriatedtungsten cathode which compriseseffecting a removal therefrom of substantially all carbon except thatwhich is combined as a thorium carbide compound by heating the cathodefor a limited time interval in pure dry hydrogen to temperaturesapproximating 1500 (J. to 2000 C. and subsequently heating the cathodein vacuo to temperatures approximating 2850 C. to effect a conversion ofthe thorium carbide to thorium metal.

5. The method of activating a thoriatedtungsten cathode which compriseseffecting a removal therefrom of substantially all carbon exce )t thatwhich is combined as a therium carbide compound by heating the cathodefor a limited time interval in pure dry hydrogen to temperaturesapproxlmating 1500 C. to 2000 C. and subsequently heating the cathode invacuo to temperatures approximating 2850 C. to effect a conversion ofthe thorium carbide to thorium metal and thereafter heating the cathodein the same high vacuo to temperatures approximating 2200 C. to diffusethe thorium to the cathode surface.

In testimony whereof, we have hereunto iubscribed our names this 18thday of May,

926. MALCOLM N. RICH.

ERNEST ANTON LEDERER.

